This chapter focuses on the double-spike calibrated thermal ionization mass spectrometry (TIMS) methods for measurement of mass dependent isotope fractionation in Se, Fe, and Cr. Current measurement precision is approximately ± 0.2 per mil on 80Se / 76Se, 56Fe / 54Fe, and 53Cr / 52Cr. Sample size requirements are 500ng, 1μg, and 250ng for Se, Fe, and Cr respectively. These measurements have been developed recently, and further improvements in precision and sample size are likely. The present purification procedures for these elements and the geochemical applications of the measurements are reviewed. TIMS instruments were usually limited to measurement of positive ions. One of the major recent developments in TIMS is the development of methods for measuring negative ions. An important future direction in mass spectrometry of Se, Fe, and Cr, is multi-collector inductively coupled plasma- mass spectrometry (MC-ICP-MS). This recent technique has a number of advantages over TIMS techniques and may eventually dominate. Instrumental discrimination is very large with MC-ICP-MS and must be monitored and modeled correctly.
